No one disputes the fact that global climate changed in the 20th Century. Those who are skeptical of the human contribution to climate change point to natural variations as the primary cause of this measured change. Separating human-induced changes from natural changes has been a challenge to climate scientists. Karoly et al. (2003) examine changes over North America over the last 100 years by dividing the 100-year record into to 50-year blocks. Global climate models generally show that increased atmospheric greenhouse gases produce (1) more warming over land than over oceans, (2) more warming in winter than summer, (3) larger rise of the nighttime daily minimum than the daytime maximum temperature, and (4) more warming at higher latitudes. They use 5 coupled ocean-atmosphere global climate models to simulate the "natural" climate (i.e., no increase in greenhouse gases) over the 20th Century and an "enhanced greenhouse gas" climate (i.e., one where effects of observed increases in greenhouse gases are included in the model). They find that over the 1900-1949 period most of the changes observed were due to natural variations, but that changes from 1950-1999 cannot be explained by natural variations but these changes are consistent with changes produced in the 5 global models where effects of enhanced greenhouse gas concentrations are included. The roles of land-use change, carbon black, and other nonsulfate aerosols are not included in these models and may lead to additional changes on the regional scale. They conclude that humans have made a detectable contribution to climate over North America in the last 50 years that cannot be explained by natural variations.
Reference
Karoly, D.J., K. Braganza. P.A. Stott, J.M Arblaster, G.A. Meehl, A.J. Broccoli, and K.W. Dixon, 2003: Detection of a human influence on North American climate. Science, 302, 1200-1203.